Commercial polyethylenes generally fall into one of two general categories based on their processability and their product properties.
Processability is the ability to predict and economically process and shape a polymer uniformly. Processability involves such elements as thermal stability, how easily the polymer flows, melt strength, and whether or not the extrudate is distortion free. Linear polyethylene (LPE) is more difficult to process than low density polyethylenes (LDPE) because LPE's are not as thermally stable, LPE's require more motor power and produce higher extruder pressures to match the extrusion rate of LDPE's. LPE's also have lower melt strength which, for example, adversely affects bubble stability during blown film extrusion, and they are prone to melt fracture at commercial shear rates. On the other hand, however, LPE's exhibit superior physical properties as compared to LDPE's.
In order to take advantage of the superior physical and mechanical properties of LPE's, expensive antioxidants and processing aids must be added to the polymer, and extrusion equipment must be modified to achieve commercial extrusion rates.
It is common practice in the industry to add low levels of an LDPE to an LPE to increase melt strength, to increase shear sensitivity, i.e., to increase flow at commercial shear rates; and to reduce the tendency to melt fracture. However, these blends generally have poor mechanical properties as compared with neat LPE.
A second technique to improve the processability of LPE's is to broaden the products' molecular weight distribution (MWD) by blending two or more LPE's with significantly different molecular weights, or by changing to a polymerization catalyst which produces broad MWD. The physical and mechanical properties of these broader MWD products are also similar to a single LPE component with equivalent weight-average molecular weight, however, the processability of these products is inferior to LDPE/LPE blends.
It is desirable in many polymerization processes, particularly a gas phase process, to use a supported catalyst. Supported metallocene-alumoxane catalysts have been described in various issued U.S. Patents. U.S. Pat. No. 4,937,217 generally describes a mixture of trimethylaluminum and triethylaluminum added to an undehydrated silica then adding a metallocene to form a dry catalyst. EP-308177-B1 generally describes adding a wet monomer to a reactor containing a metallocene, trialkylaluminum and undehydrated silica. U.S. Pat. Nos. 4,912,075, 4,935,937 and 4,937,301 generally relate to adding trimethylaluminum to an undehydrated silica and then adding a metallocene to form a supported catalyst. Similarly, U.S. Pat. Nos. 5,008,228, 5,086,025 and 5,147,949 generally describe forming a dry supported catalyst by the addition of trimethylaluminum to a water impregnated silica then adding the metallocene. U.S. Pat. No. 4,914,253 describes adding trimethylaluminum to undehydrated silica, adding a metallocene and then drying the catalyst with an amount of hydrogen to produce a polyethylene wax. U.S. Pat. Nos. 4,808,561 and 4,701,432 describe techniques to form a supported catalyst where the inert carrier, typically silica, is calcined and contacted with a metallocene(s) and a activator/cocatalyst component. U.S. Pat. No. 5,238,892 describes forming a dry supported catalyst by mixing a metallocene with an alkyl aluminum then adding undehydrated silica. Lastly, U.S. Pat. No. 5,240,894 generally pertains to forming a supported metallocene/alumoxane catalyst system by forming a metallocene/alumoxane reaction solution, adding a porous carrier, evaporating the resulting slurry to remove residual solvent from the carrier.
Traditionally, metallocene catalysts produce polymers having a narrow molecular weight distribution and a high molecular weight. Narrow molecular weight distribution polymers tend to be more difficult to process. The broader the polymer molecular weight distribution the easier the polymer is to process. Typically metallocene polymers are blended with other polymers or two or more metallocene catalysts are used or certain substituted metallocene catalysts are used to broaden molecular weight distribution.
A need exists in the industry for a metallocene catalyst and method for supporting this catalyst to produce more easily processable polymers having a broader molecular weight distribution with uniform comonomer distribution.